1. Field of the Invention
The present invention relates to an improvement of an aerostatic bearing.
2. Description of the Prior Art
In an aerostatic bearing, a guide member and a movable member are disposed such as to be opposed to each other through a bearing clearance therebetween, and one of the guide member and movable member is formed with an air-supply passage so that air flow is supplied to the bearing clearance. This aerostatic bearing has a smaller moving error and higher precision as compared with a rolling bearing due to an air-averaging effect. Therefore, the aerostatic bearings are used in various apparatus and machines requiring high precision. However, as the precision required by machine tools and measuring devices becomes higher, working precision and measuring precision as small as 1 nanometer or less are required. Therefore, micro-vibration of the aerostatic bearing, which has not been an issue until now, becomes hindrance for enhancing the precision.
The micro-vibration of the aerostatic bearing is generated when air flow is perturbed to cause a turbulent flow state. Vibration caused by the turbulent flow is vibration of 10 to several tens of kHz having no natural frequency. For this reason, resonance frequency of a machine can not be out of a band of frequency of this micro-vibration. Therefore, although amplitude of actual vibration is 1 nanometer or less, depending on the configuration of a machine, the micro-vibration may produce resonance, and the vibration may increase to several tens of nanometer. In a conventional aerostatic bearing, such micro-vibration is not negligible, and this is a problem for machines or apparatus requiring working precision and measuring precision of 1 nanometer or less as described above.
To realize a highly precise aerostatic bearing, it is necessary to eliminate a turbulent flow of air flowing through a pipe in the bearing, or to eliminate influence of vibration caused by the turbulent flow. If an air-supply pressure is merely lowered, the flow velocity becomes small, and the turbulent flow in the pipe can be eliminated. However, since the rigidity of aerostatic bearing is proportional to pressure, the aerostatic bearing can not be used under low pressure.